A skid steer loader is a compact, highly maneuverable vehicle in which the wheels on opposite sides of the vehicle are independently driven. Maneuvering is accomplished by driving the wheels on opposite sides of the vehicle at different speeds and/or in different directions to effect propulsion and steering. The operator sits in front of the engine and between a pair of hydraulically actuated loader arms on which a bucket, grapple fork, auger or other implement can be carried. Suitable controls are provided in the operator's compartment for controlling the vehicle itself and the loader arms, as well as any implement mounted on the loader.
Various factors affect the design of skid steer loaders, and particularly their frames. It will be appreciated that the frames of skid steer loaders must necessarily be of rugged construction capable of withstanding skid steering forces and loader forces. Skid steer loaders are typically operated over adverse terrain under less-than-ideal conditions, and this type of operation imposes additional loading requirements on their frames. In addition, there are considerations concerning the relationship between wheel base and track for effective skid steering, the overall height and width of the vehicle, the provision of sufficient space in the operator's compartment for the operator and controls, the location of the center of gravity, etc. which must be taken into account. The frames of skid steer loaders are thus subject to relatively severe design requirements.
One approach to the construction of frames for skid steer loaders has been to utilize a pair laterally spaced apart transmission cases, one for each side of the vehicle, interconnected by various cross members to define an operator's compartment and an engine compartment between the transmission cases. Although somewhat expensive, this approach works reasonably well; however, there are several drawbacks to the use of separate transmission cases. The primary drawback is that the overall width of the vehicle is limited by the fact that sufficient space between the wheels must be provided for the transmission cases and the operator. The overall width of a skid steer loader can be a critical factor in its operational utility, particularly at construction sites where it is necessary to maneuver through doorways or between columns. Raising the operator and positioning the transmission cases only far enough apart to provide space for the feet of the operator is not entirely satisfactory because it raises the center of gravity and the overall height of the vehicle. Positioning the transmission cases closer to the wheels results in the accumulation of more mud and the like, which in turn affects tire wear. Another drawback, which is related to clearance between the transmission cases and wheels, is that accessibility to and maintenance of the hydrostatic transmissions are more difficult. The motors are typically mounted on the inner walls of the transmission cases beneath the vehicle.
Another approach has been to provide a single transmission case for housing the transmissions for both sides of the vehicle. This approach overcomes some of the disadvantages associated with the use of separate transmission cases, however, the prior frames utilizing this approach have had other disadvantages. For example, U.S. Pat. No. 3,895,728 to Heggen shows a skid steer loader having a dual frame arrangement which substantially isolates material handling forces from the tractor body and facilitates location of the drive components in a single body compartment. The drive components are all housed with the center compartment of an inner subframe, which is substantially surrounded by an outer subframe including uprights to which the loader arms are attached. Similarly, U.S. Pat. No. 4,060,262 shows a skid steer loader wherein the transmission case is located between opposite side walls of the outer main frame so that the operator can straddle the transmission case and thereby lower the vehicle profile. Each of these devices thus utilizes two subframes, one of which also functions as the transmission case. This may have some advantages in terms of facilitating separate preassembly of the frames, however, this approach results in increased production costs because of the various brackets, locating structure, and connectors which are necessary to interconnect the subframes. The subframes are connected together by bolts, which are points of stress concentration. Additional braces, stiffeners and the like are thus required to achieve the desired rigidity. Moreover, such two-piece frames do not lend themselves well to achieving commonality or interchangeability of components between different sizes of loaders, which in turn further increases production costs.
A need has thus arisen for a new and improved frame for a skid steer loader wherein the frame consists of relatively fewer structural members formed and connected into a unitary, monocoque-type frame of greater rigidity which results in decreased production costs and increased interchangeability between models.